It is known that various polymeric materials, upon being dissolved in a solvent, are mutually incompatible. This incompatibility can have a variety of effects. For example, one polymer can flocculate out.
Alternatively, in certain systems, there is the possibility that both polymers will remain in solution but that there will be a liquid/liquid phase separation with mutually incompatible polymeric materials distributing themselves in different concentrations in the two phases.
Basic principles of such liquid/liquid phase separations and the relationship of the mutual incompatibility of the two polymers can be found in Dobry and Boyer-Kawenoki, Journal of Polymer Science, Vol. 2, No. 1, Pages 90-100, 1947.
Since the generation of such a system requires that the characteristics of three materials, namely, the two polymeric materials and the solvent, be taken into account, there has been no generalized technological use of such systems.
It is, however, possible, utilizing simple laboratory techniques, to determine suitable partners for the development of a liquid/liquid phase separation system. This will be evident from German patent document DE-OS No. 20 30 604 which describes a process for the formation of small polymer capsules.
According to the process of German patent document DE-OS No. 20 30 604, a liquid system of at least a first and a second hydrophobic polymer is formed in an organic solvent. The polymeric materials are so selected that they have, below a certain critical temperature, a homogeneous single-phase solution in the solvent. Above the critical temperature, a liquid/liquid phase separation occurs. In this case, particles of the core-forming material are dispersed in the liquid system. The system is agitated while its temperature is above the critical temperature to effect a wetting and coating of the dispersed particles of the core-forming substance by the separating phase to provide a liquid wall on each core particle. The capsules are thus grown in the liquid.
Preferably the first polymer is a halogenated rubber and the second polymer a polyethylenevinylacetate copolymer. The organic solvent is advantageously cyclohexane, toluene, xylene, carbon tetrachloride or methylisopropylketone. A similar process is described in German patent No. 1,212,497. From these disclosures, therefore, it will be evident that it is known to use such liquid/liquid phase-separated systems to produce microcapsules.
The Japanese patent publication JP 60-78 777A describes a thermocarbon ribbon which has an intermediate layer of its meltable transfer layer thermally expandable minute spheres or balls upon which the color layer is applied. This material has been found to have good printing and transfer quality even for rough papers.
However, this material is not suitable for multi-use (multi-strike) purposes and because it is necessary to provide a separate layer containing the microscopic balls, the cost of fabricating the material is high.
German patent No. 12 01 855 describes a carbon ribbon which has small droplets or microscopic balls and nonvolatile materials, a pigment or a dyestuff, distributed in a resin. This ribbon also is not suitable for multi-use or multi-strike purposes in a thermal transfer system.